1. Field of the Invention
The present invention relates to an image forming apparatus and method used for a copier, facsimile, and printer, and more particularly to an image forming apparatus having a double side image formation mode in which images are formed on both sides of a sheet and a method therefor.
2. Description of Related Art
Image forming apparatuses such as printers, copiers, and facsimiles are proposed to be of full-color, tandem type to form color images at high speed and with high quality. A typical tandem image forming apparatus includes four image forming units for yellow (Y), magenta (M), cyan (C), and black (K) disposed in parallel to each other in which toner images of yellow, magenta, cyan, and black are successively formed, transfers (primary transfer) the toner images onto an intermediate transfer belt serving as an intermediate transfer member, then collectively transfers (secondary transfer) the toner images from the intermediate transfer belt onto a transfer sheet, and fixes the toner images formed on the transfer sheet, thereby forming full-color and black-and-white (monochromatic) images.
Generally, the image forming apparatuses employ a so-called double-sided printing technique of forming images on both sides of a sheet (transfer sheet). According to this technique, when images are formed on both sides by, e.g., a full-color tandem apparatus, after a color toner image to be formed on one side (first side) of a sheet is formed on the intermediate transfer belt, the image is subjected to secondary transfer to and fixed onto the side of the sheet, the sheet is temporarily housed in a double side reversion feeding apparatus, the sheet is again fed from the double side reversion feeding apparatus while taking a timing with a color toner image on another side of the sheet formed on the intermediate transfer belt, and the another color image is subjected to secondary transfer to and fixed onto another side (second side) of the sheet. Some image forming apparatuses employ a belt transportation technique that, without using the intermediate transfer belt, transports sheets onto a transportation belt and successively transfers toner images from photoconductive drums to the sheets. Also in such apparatuses, after performing transfer and fixing to one side of a sheet, the sheet is reversed by the double side reversion feeding apparatus, and toner images are successively transferred and fixed to another side of the sheet transported on the transportation belt, whereby images can be formed on both sides.
In such conventional image forming apparatuses forming images on both sides, when a color image is formed on each side of a sheet, secondary transfer (or transfer to sheets transported on the transportation belt) to the sheet from the intermediate transfer belt and fixing of toner images must be performed for each side, and the sheet must be temporarily housed in the double side reversion feeding apparatus. For this reason, excessively long processing time has been problematic. Accordingly, an image forming apparatus forwardly and backwardly rotating photoreceptors to form images on both sides is adopted which forms an image on the back when the photoreceptors are switched from forward rotation to backward rotation, thereby reducing time for switching between recording of a first image and recording of a second image (see, e.g., Patent Reference 1).
[Patent Reference 1] Japanese Published Unexamined Patent Application No. Hei 5-88479 (pages 2 and 3, FIG. 1)
In recent years, a study has been made of the evolution of full-color image forming apparatuses employing the electrophotographic system as an alternative to offset printing used in the publication industry and the like. In the evolution of such full-color image forming apparatuses, high image quality comparable to that of offset printing is demanded, and the formation of images on both sides of a sheet requires addressing new problems that have been conventionally trivial.
FIGS. 8A and 8B are diagrams for explaining problems in the formation of images on both sides of a sheet. FIG. 8A shows an image formation state on a first side, and FIG. 8B shows an image formation state on a second side. In FIGS. 8A and 8B, register marks used in the printing industry and the like are formed. The register marks are widely used to correctly align multicolored images or as cutoff lines, for example. Image forming apparatuses generally perform positioning with reference to the front edge of a sheet transported to form images. For example, if a register mark is specified to be formed at a distance of a [mm] from the front edge, the register mark can be formed almost a [mm] away from the front edge, though somewhat different depending on machines. On the other hand, the distance b [mm] between the rear edge of a sheet and a register mark cannot be correctly determined because of an error (e.g., ±1 mm) of sheet lengths, a belt speed error during image writing, and variations in sheet transportation speeds.
As shown in FIG. 8B, in cases where images are formed on both sides by reversing sheets, the reversion of a sheet turns the front edge of a second side thereof in a sheet transportation direction into the rear edge of a first side thereof. As a result, if no adjustment is made, the position of register marks (dashed lines shown in FIG. 8B) formed on the back (first side) of an image formation side (second side) shown in FIG. 8B is b [mm] from the front edge, causing a difference of b−a [mm] with the position (distance a [mm]) of register marks formed on the surface of the image formation side shown in FIG. 8B. If the register marks do not match positionally, a positional mismatch between the surface and the back will cause great problems such as a reference position that cannot be determined when sheets are cut at their ends, and column mismatch between opened pages during bookbinding and the like.
In this case, if the distance b [mm] is a value correctly found, the surface and the back can be registered by adjusting the timing of forming an image on the second side. However, an error of sheet lengths cannot be determined in advance. Particularly if a sheet is allowed to pass through a fixing unit to fix toner images, the sheet stretches by a prescribed length because the sheet is heated while being pressed, and the stretch changes depending on the moisture content of the sheet. Therefore, it is difficult to control registration in advance. For this reason, with the prior arts, it has been impossible to register the surface and the back of sheets with great precision.